Squeegee for stencil printing

ABSTRACT

A squeegee for spreading a conductive paste across a stencil during a stencil printing process is provided. The stencil covers an underlying base such that the conductive paste is spread across the stencil and into openings in the stencil to contact the underlying base. The squeegee has a leading surface at the front of the squeegee, a trailing surface at the rear of the squeegee, and a bottom surface. The bottom surface is oriented at an oblique angle relative to the leading surface and the trailing surface. In some embodiments, a rear corner or edge of the squeegee is the contact point between the squeegee and the stencil. Some of the paste can be collected beneath the bottom surface and scraped along with the squeegee.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to application Ser. No. 17/410,273,titled FLATTENING SURFACE OF PASTED TRACK IN STENCIL PRINTING PROCESS,filed on the same day as the present application. The presentapplication is also related to application Ser. No. 17/410,312, titledMETHOD OF MANUFACTURING A CONDUCTIVE TRACK ON A BOARD VIA STENCILPRINTING, filed on the same day as the present application. The presentapplication is also related to application Ser. No. 17/410,381, titledSTENCIL FOR STENCIL PRINTING PROCESS, filed on the same day as thepresent application. Those applications are incorporated herein byreference in their entirety, but they are not admitted to be prior artwith respect to the present application by mention in thiscross-reference section.

TECHNICAL FIELD

The present disclosure relates to a squeegee for spreading conductivepaste on a board for manufacturing a conductive track on a board viastencil printing.

BACKGROUND

For printed circuit board (PCB) applications, and others, tracks ofconductive paste can be printed on the board using stencils. Stencilprinting is the process of depositing paste (e.g., solder paste) on anunderlying board to establish electrical connections. For example, astencil can be layered on top of the board, and paste can be providedinto the holes of the stencil and thereby onto the board using asqueegee. The stencil can then be removed from the underlying board,leaving behind the paste in a desired shape as dictated by the patternon the stencil. The paste can then be cured, creating a hardenedconductive track configured to electrically connect multiple componentson the board.

SUMMARY

According to an embodiment, a system for spreading a conductive pasteacross a stencil to apply the conductive paste to an underlying base ina stencil printing process is provided. The system includes a basehaving an upper surface, and a stencil removably covering at least aportion of the upper surface of the base. The stencil has a lowersurface configured to removably contact the upper surface of the base.The stencil has an upper surface, and defines an opening extendingtherethrough from the upper surface of the stencil to the lower surfaceof the stencil. A squeegee is configured to spread a conductive pasteacross the upper surface of the stencil and force the conductive pastethrough the opening and onto the upper surface of the base. The squeegeehas a leading surface, a trailing surface, and a bottom surfaceconnecting the leading surface to the trailing surface. The bottomsurface is oriented at an oblique angle relative to the leading surfaceand the trailing surface.

According to an embodiment, a squeegee configured for use in a stencilprinting process to spread a conductive paste across a stencil and ontoan underlying base is provided. The squeegee has a leading surfaceconfigured to face a direction of travel as the squeegee is spreadacross the stencil. The squeegee has a trailing surface configured toface away from the direction of travel. The squeegee has a bottomsurface connecting the leading surface to the trailing surface, whereinthe bottom surface is oriented at an oblique angle relative to theleading surface and the trailing surface.

According to an embodiment, a method of manufacturing a conductive trackon a board via stencil printing is provided. The method includes placinga stencil over at least a portion of an underlying base; applying aconductive paste in an opening of the stencil such that the conductivepaste contacts the underlying base; and scraping a corner of a squeegeeacross the stencil to remove at least some of the conductive paste fromthe stencil while keeping some of the conductive paste in the openingand contacting the underlying base. The corner of the squeegee is at anintersection of a bottom surface of the squeegee and a trailing surfaceof the squeegee such that during the scraping some of the conductivepaste is collected beneath the bottom surface of the squeegee andforward of the corner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an embodiment of a top perspective view of a paste beingspread toward an opening in a stencil and onto a base substrate, andFIG. 1B shows a side view of the same.

FIG. 2A shows an embodiment of a top perspective view of a squeegeeadvancing such that the paste is spread into the opening in the stenciland onto the base substrate, and FIG. 2B shows a side view of the same.

FIG. 3A shows an embodiment of a top perspective view of the squeegeeadvancing further and no longer above the opening in the stencil, andFIG. 3B shows a side view of the same.

FIG. 4 shows a side view of the stencil being removed from the basesubstrate in a vertical direction with some of the paste being draggedalong, according to an embodiment.

FIG. 5 shows a final shape of the paste on the base substrate with thestencil removed, according to an embodiment.

FIG. 6A shows a side view of a squeegee moving across an opening in astencil, and FIG. 6B shows an enlarged view of a region of FIG. 6Ahighlighting the paste material being dragged along behind the squeegeedue to surface tension, according to an embodiment.

FIG. 7A shows a side view of the squeegee moving further across theopening of FIG. 6A, and FIG. 7B shows an enlarged view of a region ofFIG. 7A highlighting residual paste left on the stencil, according to anembodiment.

FIG. 8A shows a side view of the stencil of FIG. 7A being removed in avertical direction, and FIG. 8B shows the stencil being removed furtherstill, according to an embodiment.

FIG. 9 shows the final shape of the paste after the stencil is removed,highlighting an end-of-track bump, according to an embodiment.

FIG. 10A shows a comparison of side views of the final shape of thepaste in both a base case with the stencil removed at a first speed, anda faster-removal case with the stencil removed at a second speed fasterthan the first speed, according to an embodiment. FIG. 10B shows anenlarged region of the comparison of FIG. 10A, showing the differencesin the end-of-track bumps.

FIGS. 11A-11D illustrated various shapes of the bottom surface of thesqueegee according to embodiments.

FIG. 12A shows the squeegee of FIG. 11B spreading paste material over astencil according to an embodiment. FIG. 12B shows the squeegee furtheradvanced along the stencil, and FIG. 12C shows an enlarged region ofFIG. 12B showing no residual paste left on the stencil, according to anembodiment.

FIGS. 13A-13C show similar views as FIGS. 12A-12C, except now using thesqueegee of FIG. 11C according to an embodiment.

FIGS. 14A-14C show similar views as FIGS. 12A-12C, except now using thesqueegee of FIG. 11D according to an embodiment.

FIG. 15 shows a comparison of the final shape of the paste after thesqueegees of FIGS. 11A-11D are used and the stencils are removed,according to embodiments.

FIG. 16A shows a stencil having a stepped edge according to anembodiment; FIG. 16B shows an enlarged region of FIG. 16A.

FIG. 17A shows a stencil having a stepped edge according to anotherembodiment; FIG. 17B shows an enlarged region of FIG. 17A.

FIG. 18A shows a comparison of the final shape of the paste after usinga stencil having no stepped edge, another stencil having a stepped edgeaccording to the embodiment in FIG. 16A, and another stencil having astepped edge according to the embodiment in FIG. 17A. FIG. 18B shows anenlarged view of the end-of-track bumps of the comparison.

FIGS. 19A-19D show stencils having stepped edges according to variousembodiments.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the embodiments. Asthose of ordinary skill in the art will understand, various featuresillustrated and described with reference to any one of the figures canbe combined with features illustrated in one or more other figures toproduce embodiments that are not explicitly illustrated or described.The combinations of features illustrated provide representativeembodiments for typical applications. Various combinations andmodifications of the features consistent with the teachings of thisdisclosure, however, could be desired for particular applications orimplementations.

Conductive pastes can be applied to an underlying base substrate byusing a stencil. The stencil may be placed over the underlying basesubstrate, and the conductive paste can be spread over the stencil whereit comes into contact with the underlying base substrate via holes inthe stencil. This process can be utilized for printed circuit board(PCB) applications, and others including fuel cells, electrodeassemblies, and the like. Once cured, the conductive paste hardens toform a conductive track between two electrical components on the basesubstrate (e.g., the PCB board).

FIGS. 1-5 show a process of applying a paste to a PCB board utilizing astencil, according to an embodiment. FIGS. 1A and 1B show an initialapplication of a conductive paste 10 to apply a pasted track to anunderlying base substrate, or base 12. The paste 10 can be a glue oradhesive containing silver or other conductive material that willultimately, once cured, form a conductive path between components on theunderlying base 12. The base 12 may be a PCB board, a ceramic, polymer,glass, or a conductive material. A stencil 14 is placed over the base12. The stencil 14 has an aperture or opening 16 through which the paste10 can come into contact with the base 12. The stencil may have manyopenings and of differences sizes and shapes to create a desired patternof electrical conduits printed on the base 12. A squeegee 18 is used tomove horizontally to spread and scrape the paste 10 across an uppersurface of the stencil to fill the opening 16 with the paste 10.

FIGS. 1A and 1B show the squeegee 18 moving across the stencil 14 in adirection toward the opening 16. FIGS. 2A and 2B show the squeegee 18moved to a position such that the paste 10 has started to fill theopening 16. The squeegee 18 then continues to move across the uppersurface of the stencil 14, scraping and leveling the paste 10 so that itis intended to be level and take the shape of the opening 16. FIGS. 3Aand 3B show the final shape and outlay of the paste 10 within thestencil 14 with the squeegee removed from the stencil 14.

With the opening 16 of the stencil 14 filled with the conductive paste10, the stencil 14 can be removed. The stencil 14 can be removed in avertical direction away from an upper surface 20 of the base 12, asindicated by arrow 22. This removes the stencil 14 from the base 12,leaving behind the 10 on the upper surface 20 of the base 12. Thesmearing move the squeegee 18 (not shown in FIG. 4 ) causes some of thepaste 10 to be left on an upper surface of the stencil 14, as indicatedat 24. Also, due to and during the removal of the stencil 14 from thebase 12, a residual amount of the paste 10 is lifted upward and draggedalong with the stencil 14, as indicated at 26, and eventually falls backto the base 12 due to gravity and/or surface tension. The final shape ofthe paste 10 is shown in FIG. 5 . This illustrates the shape of thepaste 10 once the stencil 14 has been fully removed and the residualpaste has fallen back down to the base 12.

Due to the smearing action of the squeegee 18 and the removal action ofthe stencil 14, the paste 10 assumes a shape having flat region orplateau region 28, as well as a bump at the end of the paste 10. Thisbump 30 can be referred to as an end-of-track bump 30. The end-of-trackbump 30 can cause an undesirable non-contact zone between the pastedtrack and other parts that are later added to the base 12. Therefore,according to various embodiments disclosed herein, a system and methodfor reducing the size of this end-of-track bump 30 is provided. In someembodiments, the end of the squeegee that contacts the paste isspecifically shaped to reduce the size of the end-of-track bump. In someembodiments, the stencil is lifted at specific speeds to reduce the sizeof the end-of-track bump. In some embodiments, the stencil isspecifically shaped to reduce the size of the end-of-track bump. And insome embodiments, these concepts are combined to further reduce the sizeof the end-of-track bump. The designs and methods disclosed herein areto improve the surface flatness of the paste by removing or reducing thesize of the end-of-track bump 30 such as the one shown in FIG. 5 .

FIGS. 6-9 described below illustrate a “base case,” which is compared tothe later-described improvements to the squeegee design, the stencilremoval speed, and the stencil design. In the base case, FIGS. 6-9 showa typical stencil printing process and the generation of an end-of-trackbump in the deposited paste. As described in the embodiments illustratedin FIGS. 1-5 , the paste 10 is printed onto a base 12 by use of astencil 14 with an opening 16 therethrough. A squeegee 18 applies thepaste 10 to the base via the opening 16. FIG. 6A shows the squeegee 18moving horizontally to scrape excess paste from the stencil 14. Thisstep can be performed as a subsequent step after the paste 10 is alreadyapplied (as shown in FIG. 6A), or as a single step when the paste isapplied (as shown in FIGS. 1-3 ).

In the base case, the squeegee 18 is rectangular shaped, havinggenerally perpendicular corners, the squeegee 18 moves horizontallyacross the stencil 14 at a speed of 40 millimeters (mm) per second (s),the opening 16 is 4 mm wide, and the stencil thickness (in the verticaldirection in FIG. 6A) is 0.1 mm. This is merely an example, and is usedas the control to compare to the other designs and embodiments describedherein.

Referring to FIGS. 6A-6B, as the squeegee 18 pushes the paste 10 forward(e.g., to the right in the Figures), some of the paste at 32 “attaches”to a bottom surface 34 of the squeegee 18. This is due to the surfacetension force of the fluidic paste material forcing some of the paste tofollow along with the bottom surface 34, causing the visible ridge inthe paste at 32.

Referring to FIGS. 7A-7B, the squeegee 18 is shown completely moved outof the opening 16 of the stencil 14. However, there is still someresidual paste at 36 located on an upper surface 38 of the stencil 14.There is also some residual paste at 40 that is beginning to form theend-of-track bump discussed above. The paste left at 36 and 40 is atleast partially due to the surface tension force of the fluidic pastematerial dragging along with the squeegee 18 as described above, andthen being left behind as the squeegee 18 is moved beyond the opening16.

FIG. 8A shows an initial separation of the stencil 14 from theunderlying base 12, and FIG. 8B shows a subsequent further separation ofthe stencil 14 from the underlying base 12. It can be seen that thereremains some paste “attached” or held in close contact with the uppersurface 38 of the stencil 14 and a side surface 42 of the stencil 14. Asthe stencil 14 is removed further from the base 12, some of the pastethat remains held close to the surfaces 38, 42 drips and falls back tothe paste 10 on the base 12. And, as shown at 44, due to the viscosityor thickness of the paste 10, after the stencil 14 moves far enough fromthe base 12, some of the residual paste 10 on the base 12 may be liftedin a vertical direction away from the base 12 and subsequently snap backto the base 12 once the paste breaks away and is completely separatedfrom the stencil 14. The break in the paste is shown at 45, althoughthis break can happen on either or both sides of the stencil 14. All ofthis culminates in ultimately forming the end-of-track bump 46 in thefinal shape of the paste 10 on the base 12 (as shown in FIG. 9 ) oncethe stencil 14 is completely removed from the base 12. The morphology ofthe deposited track of paste 10 does not have optimum flatness due tothis end-of-track bump 46.

In all, the end-of-track bump 46 is formed from the following: (1) thesurface tension of the paste material causing attachment or hugging ofthe paste to the edges of the squeegee, which is then dragged onto anddeposited onto the stencil; (2) the stencil plate removal processapplies a vertical shear force to the deposited track, and the pastethat is attached to the stencil falls back to the pasted track on thebase; (3) under the effects of surface tension and viscosity, the pastedtrack—now with the excess paste—gradually forms the end-of-track bump46.

The embodiment described with reference to FIGS. 6-9 can be referred toas the base case or control case. This base case is compared to othertechniques, structures, methods, and results described below withreference to subsequent Figures.

Stencil Removal Speed

In one embodiment, the stencil is removed from the underlying base at anincreased speed to reduce the size of the end-of-track bump, therebyimproving the flatness of the pasted track. Since the stencil removalprocess applies a vertical shear force to the pasted track, the stencilremoval speed can affect the final shape of the deposited track; afaster speed is noticed to increase the shear force. Therefore, thestencil was tested at a vertical removal speed of 1,000 mm/s. FIG. 10Acompares the pasted track of base case (e.g., the pasted track of FIG. 9resulting from a stencil removal speed of 40 mm/s) with the pasted trackthat results from an increase in stencil removal speed to 1,000 mm/s.FIG. 10B shows an enlarged view of the end-of-track bumps that resultfrom each stencil removal. The stencil removal speed of 1,000 mm/sresulted in a 19% reduction in the height of the end-of-track bump 48compared to the base case end-of-track bump 46.

This process was repeated for different stencil removal speeds, namely aremoval speed of 120 mm/s, 200 mm/s, and 400 mm/s. The removal speed of120 mm/s was noticed to not have a substantial impact on the overallend-of-track bump height, while the removal speeds of 200 mm/s and 400mm/s resulted in a similar reduction in height as the removal speed of1,000 mm/s (e.g., approximately 19% reduction in height). The jump inbump-height reduction from using a 120 mm/s removal speed to a 200 mm/sremoval speed may indicate that a sufficiently fast speed can moreeasily break the residual material attached to the upwardly-movingstencil, thus contributing to a reduction in the amount of paste left inthe end-of-track bump. According to an embodiment therefore, a stencilremoval speed of greater than 200 mm/s can be utilized to improve theend-of-track bump in the resulting shape of the pasted track. In yetfurther embodiments, a stencil removal speed of between 200 mm/s and1,000 mm/s is utilized. In yet further embodiments, a stencil removalspeed of 400 mm/s is utilized.

Squeegee Design

To reduce the presence and amount of residual paste dragged along withand attached to the squeegee 18 (described above), various embodimentsof squeegee designs will now be described. The squeegee designs utilizedifferent shapes at the contacting ends that slide along the stencil 14to spread and scrape the paste. FIG. 11A shows the squeegee 18 describedabove with reference to FIGS. 6-7 . This squeegee design can be referredto as the base design or control design. The squeegee 18 has arectangular shape with a leading surface 50 (also referred to as a frontsurface), a bottom surface 52, and a trailing surface 54 (also referredto as a rear surface or back surface). The leading surface 50 intersectsthe bottom surface 52 at a generally perpendicular angle. Likewise, thetrailing surface 54 intersects the bottom surface 52 at a generallyperpendicular angle. The intersection of the leading surface 50 and thebottom surface 52 defines a leading corner or leading edge 56 whichdefines the lowest point of the squeegee 18 when the squeegee 18 is heldat an angle (such as the angle shown in FIGS. 6-7 ) while scrapingacross the stencil 14. The leading edge 56 is not only the lowest pointof the squeegee 18 when in use, but also the forward-most point of thelower surface 52 when in use in the orientation shown, and every pointrearward of the squeegee 18 is located further from the stencil 14. Thiscombination can lead to the phenomena of attachment of paste to thesqueegee 18 described above.

FIGS. 11B-11D illustrate various embodiments of squeegee designs thatare specifically shaped to address the issue of the residual paste beingdragged along with the squeegee in order to reduce or eliminate theultimate end-of-line bump.

In the embodiment illustrated in FIG. 11B, the squeegee 18′ has aleading surface 60, a bottom surface 62, and a trailing surface 64. Thebottom surface 62 is angled oblique relative to the leading surface 60and trailing surface 64. The angles between the bottom surface 62 andthe leading and trailing surfaces 60, 64 are such that the bottomsurface 62 can be parallel to the upper surface of the stencil as thesqueegee 18′ is smearing and scraping the paste. In one embodiment, theangle between the bottom surface 62 and the leading surface 60 isbetween 110 degrees and 140 degrees, and in an embodiment the angle isbetween 120 degrees and 130 degrees. Conversely, in an embodiment, theangle between the bottom surface 62 and the trailing surface 64 isbetween 40 and 70 degrees, and in an embodiment the angle is between 50and 60 degrees. The embodiment of the squeegee 18′ illustrated in FIG.11B can be referred to as the “flat” design because the bottom surface62 can be engaging the stencil in a flat, parallel, face-to-facerelationship as the squeegee 18′ is moved across the stencil. Thisoperation is shown in FIG. 12 .

In the embodiment illustrated in FIG. 11C, the squeegee 18″ has aleading surface 70, a bottom surface 72, and a trailing surface 74. Thebottom surface 72 is angled oblique relative to the leading surface 70and the trailing surface 74. The angles between the bottom surface 72and the leading and trailing surfaces 70, 74 are such that theintersection of the bottom surface 72 and the trailing surface 74 (e.g.,corner or edge 76) is the point of contact with the paste and stencil asthe squeegee 18″ moves along the stencil. This operation is shown inFIG. 13 . The embodiment of the squeegee 18″ illustrated in FIG. 11C canbe referred to as the “front cut” design because the front of thesqueegee 18″ (i.e., the leading edge) is “cut” such that it is the rearpart of the squeegee 18″ that pushes the paste. In one embodiment, theangle between the bottom surface 72 and the leading surface 70 isbetween 135 degrees and 165 degrees, and in an embodiment the angle isbetween 145 degrees and 155 degrees. Conversely, in an embodiment, theangle between the bottom surface 72 and the trailing surface 74 isbetween 15 and 45 degrees, and in an embodiment is between 25 and 35degrees. Through tests using the method shown in FIGS. 13A-13C, thisangle resulted in a proper amount of conductive paste being pushedbeneath the bottoms surface 72 so as to result in a reduced end-of-trackbump.

In the embodiment illustrated in FIG. 11D, the squeegee 18′″ has aleading surface 80 a bottom surface 82, and a trailing surface 84. Thebottom surface 82 is rounded. The bottom surface 82 may be semi-circularin shape. The rounded shape of the bottom surface 82 assures that thereis always a single point of contact between the bottom surface 82 andthe underlying stencil during scraping of the squeegee 18′″ no matterthe angle of orientation of the squeegee 18″ ‘.

FIGS. 12-14 show the squeegees of FIGS. 11B-11D, respectively, inoperation. Referring to FIG. 12A, the flat design squeegee 18’ of FIG.11B is shown. The squeegee 18′ is shown scraping across the stencil(e.g., to the right in the view shown), smearing the paste 10 andscraping the paste. FIG. 12B shows a progression of the squeegee 18′,and FIG. 12C shows an enlarged region of where the opening 16 ends.Clearly, due to the flat design as compared to the base design, lesspaste builds up on the trailing side of the squeegee. This can be due tothe bottom surface 62 being flat compared to the underlying paste andthe upper surface of the stencil 14. Further, since the bottom surface62 is flat and smooth relative to upper surface 38 of the stencil 14,the bottom surface 62 can wipe paste off cleanly with the upper surface38 due to the face-to-face relationship. Less paste is found at the endof the opening 16 as compared to the base case, which ultimately leadsto less of an end-of-track bump. Little or zero paste is left on theupper surface of the stencil 14, which results in less paste flowingback down to the base when the stencil 14 is removed.

FIGS. 13A-C show similar images using the front cut design squeegee 18″.As can be seen in these images as well, the amount of residual pasteleft at the end of the opening 16 is reduced compared to the base case.As can be seen in these figures, a gap located vertically between thesqueegee 18″ and the stencil 14, and forward of the corner 76, collectssome of the conductive paste as the squeegee 18″ is scraped across thestencil 14. The gap fills with paste as the squeegee moves across theupper surface of the stencil.

FIGS. 14A-C show similar images using the round design squeegee 18′″.The round design squeegee 18′″ has a slight amount of residual paste 10being dragged along or adhered to the trailing surface and round bottomsurface. This leads to a slight amount of paste being deposited onto theupper surface 38 of the stencil 14 after the squeegee 18′″ has finishedscraping across the stencil 14.

FIG. 15 shows a comparison of the final shape of the paste 10 afterutilizing each of the squeegee 18, 18′, 18″, and 18″ ‘. It is noted thatboth the flat design squeegee 18’ and the round design squeegee 18′″each reduced the final height of the end-of-track bump 46 by roughly 5%compared to the base squeegee 18. The front cut design squeegee 18″increased the height of the end-of-track bump 46 by roughly 6% comparedto the base squeegee 18.

In each of the embodiments of the squeegee designs described above, thesqueegee may have a thickness of less than 1 mm. In other embodiments,the thickness of the squeegee is greater than 1 mm. The thickness refersto the distance between the leading surface and the trailing surface. Inparticular embodiments, the squeegee has a thickness of between 0.1 mmand 0.5 mm. The squeegee may rubber, plastic, or other syntheticmaterial, or may be metal. The squeegee may embody a blade.

Stencil Design

Since the end-of-track bump described above is at least partially causedby the vertical shear force introduced by removal of the stencil 14, theshear force may be reduced if the paste material is away from thevertical side surface 42 of the stencil 14. Therefore according toembodiments, the design of the stencil 14 is altered to allow theremoval of more paste from the side surface 42 such that less pastestays in contact with the side surface 42 during vertical removal of thestencil 14, and thus less paste is pulled back or fallen down to thebase 12 as the stencil 14 is removed.

FIGS. 16A and 16B show a stencil 90 according to one embodiment. Thestencil 90 has a stepped edge feature to allow a gradual or brokentransition from the paste 10 to the stencil 90 as the squeegee 18′ isslid across the stencil. The stencil 90 has a first sidewall 92, a floor94, a second sidewall 96, and an upper surface 98. The sidewalls 92, 96are shown to intersect the floor 94 at right angles, although in otherembodiments other angle are utilized. The floor 94 connects the firstsidewall 92 from the second sidewall 96. The floor 94 provides a landingzone for some residual paste 10 to be delivered to as the squeegee 18′is scraped across the upper surface 98 of the stencil 90.

In the embodiment of FIGS. 16A-16B, the thickness of the entire stencil90 from the bottom surface to the upper surface 98 is 0.1 mm. Of course,the teachings of this disclosure can be applied to stencils having athickness of greater than 0.1 mm, and the dimensions explained hereinare merely exemplary. The length of the floor 94 from the first sidewall92 to the second sidewall 96 is 1 mm. The height of the second sidewallis 0.05 mm; thus it can be said that the height or depth of the stepfeature of the stencil 90 is 0.05 mm. The first sidewall 92 is spacedfrom a sidewall 99 of the stencil 90 that defines the beginning of theopening 16 by 4 mm. The length of the entire opening 16 is 5 mm, fromthe sidewall 99 to the second sidewall 96.

FIGS. 17A and 17B show a stencil 100 with a stepped edge of anotherembodiment. In this embodiment, the stencil 100 has a first sidewall102, a floor 104, a second sidewall 106, and an upper surface 108, eachof which is similar to the embodiment of FIGS. 16A-16B except differentin size. For example, in this embodiment, the length of the floor 104from the first sidewall 102 to the second sidewall 106 is 1 mm, theheight of the first sidewall 102 is 0.01 mm, and the height of thesecond sidewall 106 is 0.09 mm, combining to define an overall thicknessof the stencil 100 of 0.1 mm once again. It can thus be said that theheight or depth of the step feature of the stencil 100 is 0.09 mm. Thesecond sidewall 106 in this embodiment is taller than the secondsidewall 96 of the previous embodiment. This allows more paste 10 togather and be collected above the floor 104 while the squeegee 18″ isscraped across the upper surface 108 of the stencil 100.

Once the squeegee 18″ is done spreading and scraping the paste 10 intothe opening and across the upper surface of the stencils, the stencilswith their stepped edges can be vertically removed from the respectiveunderlying bases. FIG. 18A illustrates a comparison of the final shapeof the paste on its underlying base once the stencils are removed. Theupper paste is the base case, using no stepped edge. The middle paste isafter using the stencil 90 of FIG. 16 , and the lower paste is afterusing the stencil 100 of FIG. 17 . FIG. 18B shows an enlarged region ofFIG. 18A, highlighting the end-of-track bumps left on each track ofpaste. The end-of-track bump left on the paste is significantly smallerin each embodiment having a stepped edge compared to the base case withno stepped edge. And, the embodiment of the stencil of FIG. 17 resultsin a smaller end-of-track bump than the embodiment of the stencil ofFIG. 16 .

FIGS. 19A-19D show four additional embodiments of stencils with steppededges to assist in reducing or eliminating the end-of-track bump. Ineach of these embodiments, a constant step height of 0.05 mm is used,and a constant thickness of the stencil 101 In other words, in eachembodiment the stepped edge includes a first sidewall 110, a floor 112,and a second sidewall 114, and in each embodiment the height of thesecond sidewall 114 is the same. The difference in these embodiments isthe length of the floor 112. In the embodiment of FIG. 19A, the lengthof the floor 112 (i.e., from the first sidewall 110 to the secondsidewall 114) is 1 mm. In the embodiment of FIG. 19B, the length of thefloor 112 (i.e., from the first sidewall 110 to the second sidewall 114)is 0.7 mm. In the embodiment of FIG. 19C, the length of the floor 112(i.e., from the first sidewall 110 to the second sidewall 114) is 0.4mm. And, in the embodiment of FIG. 19D, the length of the floor 112(i.e., from the first sidewall 110 to the second sidewall 114) is 0.1mm.

It has been realized through tests that the length of the floor 112 canalter the end-of-track bump that eventually forms on the paste. Theembodiments of FIGS. 19A-19B can reduce the size of the end-of-trackbump by around 40%. The embodiment of FIG. 19C can reduce the size ofthe end-of-track bump by about 48%. And, the embodiment of FIG. 19D canreduce the size of the end-of-track bump by about 31%. Therefore, even asmall stepped edge with a 0.1 mm floor length can have significantbenefits to the size of the later-formed end-of-track bump when thestencil is removed.

It should be understood that the embodiments described herein can becombined to form other embodiments. For example, each of the modifiedsqueegee designs can be combined with a respective one of the steppededge stencil design to result in even more size reduction in theend-of-track bump.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, to the extentany embodiments are described as less desirable than other embodimentsor prior art implementations with respect to one or morecharacteristics, these embodiments are not outside the scope of thedisclosure and can be desirable for particular applications.

What is claimed is:
 1. A system for spreading a conductive paste acrossa stencil to apply the conductive paste to an underlying base in astencil printing process, the system comprising: a base having an uppersurface; a stencil removably covering at least a portion of the uppersurface of the base, the stencil having a lower surface configured toremovably contact the upper surface of the base, the stencil having anupper surface, and the stencil defining an opening extendingtherethrough from the upper surface of the stencil to the lower surfaceof the stencil; and a squeegee configured to spread a conductive pasteacross the upper surface of the stencil, wherein the squeegee has aleading surface, a trailing surface, and a bottom surface connecting theleading surface to the trailing surface, wherein the bottom surface isoriented at an oblique angle relative to the leading surface and thetrailing surface; wherein the bottom surface and the trailing surfaceintersect at a corner that defines a lower-most point of the squeegee;wherein the bottom surface and the trailing surface intersect at anangle between 15 degrees and 45 degrees.
 2. The system of claim 1,wherein the bottom surface is angled relative to the leading surface andthe trailing surface such that the bottom surface is parallel to theupper surface of the stencil and the leading surface is oblique to theupper surface of the stencil as the squeegee spreads the conductivepaste across the upper surface of the stencil.
 3. The system of claim 2,wherein the bottom surface and the leading surface intersect at an anglebetween 110 degrees and 140 degrees.
 4. The system of claim 1, wherein agap is located vertically between the squeegee and the upper surface ofthe stencil, and located forward of the corner, such that the gap fillswith paste as the squeegee moves across the upper surface of thestencil.
 5. The system of claim 1, wherein a remainder of the bottomsurface other than the lower-most point of the squeegee is verticallyhigher than the lower-most point of the squeegee such that the remainderof the bottom surface does not contact the upper surface of the stencilas the squeegee spreads the conductive paste across the stencil.
 6. Thesystem of claim 1, wherein the stencil has a stepped edge connecting thelower surface of the stencil to the upper surface of the stencil at aborder of the opening.
 7. The system of claim 6, wherein the steppededge includes a first sidewall, a floor extending transverse from thefirst sidewall, and a second sidewall extending transverse from thefloor.
 8. The system of claim 7, wherein the first sidewall is shorterthan the second sidewall.
 9. The system of claim 8, wherein the firstsidewall extends upward from the lower surface of the stencil, and thesecond sidewall extends downward from the upper surface of the sidewall.10. A squeegee configured for use in a stencil printing process tospread a conductive paste across a stencil and onto an underlying base,the squeegee comprising: a leading surface configured to face adirection of travel as the squeegee is spread across the stencil; atrailing surface configured to face away from the direction of travel;and a bottom surface connecting the leading surface to the trailingsurface, wherein the bottom surface is oriented at an oblique anglerelative to the leading surface and the trailing surface; wherein thebottom surface and the trailing surface intersect at an angle of between40 degrees and 70 degrees.
 11. The squeegee of claim 10, wherein theangle is between 50 degrees and 60 degrees.
 12. The squeegee of claim10, wherein the bottom surface and the trailing surface intersect at acorner that defines a lower-most point of the squeegee.